Recent years have witnessed a mounting interest in the RIM technology, that is a technology by which an automotive bumper, for instance, is manufactured by simultaneous polyurethane-forming reaction and molding of the resulting resin in a metal mold, and several versions of such technology have been disclosed in, inter alia, U.S. Pat. Nos. 4,254,069, U.S. Pat. No. 3,838,076, U.S. Pat. No. 4,216,543, U.S. Pat. No. 4,246,363 and U.S. Pat. No. 4,269,945.
However, the technology of reacting a polyalkylenepolyol with a polyisocyanate in a mold cavity to give a polyurethane molding in situ not only encountered various difficulties due to the relatively poor heat resistance and coatability of the polyurethane but entailed a high production cost. For this reason, attention was paid to polyurea resins which are more heat-resistant and there has been proposed a technology for producing polyurea moldings which are superior to polyurethane moldings in mechanical strength, heat resistance and other physical properties. This technique comprises reacting a polyoxyalkylenepolyamine having an average molecular weight of at least 1500, an amine terminal chain extender and a polyisocyanate in a closed mold. A method in this category is disclosed in Japanese Patent Application Laid-open No. 188643/83.
One of the important advantages derived from the use of polyurea moldings as automotive parts, such as bumpers, resides in the fact that polyurea parts may be attached to an automotive body before the entire assembly is finished with a thermosetting paint to reduce the number of necessary steps. This may be performed by attaching polyurea exterior parts to the automotive steel body, applying the same thermosetting paint onto the polyurea parts as well as the steel body, and then baking the paint film in all areas simultaneously. Therefore, it is required that the polyurea moldings are not thermally deformable at the baking temperature of conventional automotive coating lines. However, improvements in heat resistance may generally decrease toughness and impact strength, particularly at lower temperatures.
Japanese Patent Application Laid-open No. 143611/89 discloses as a solution to this problem the use of an isocyanate prepolymer produced by reacting a polyisocyanate compound with a hydroxy-, carboxyl-, amino-, or epoxy- terminated liquid polybutadiene in the polyurea RIM. This method, however, suffers from certain defects that the starting liquid polybutadiene is relatively expensive and also too viscous. These defects limit its formulation in the RIM compound to a lower proportion.
Accordingly, a need exists for a polyurea RIM composition which is cost effective and has an improved impact strength while retaining good moldability. The present invention meets this need.